The present invention relates generally to a combined gas/steam turbine power plant having a back pressure turbine.
In power plants, a steam turbine is normally driven by steam at a generally constant pressure. The live steam pressure is typically kept constant by controlling firing of a steam boiler. In the event that the steam turbine operates with back pressure, it follows that the back pressure is regulated by inlet valves of the steam turbine. Such a mode of operation can also be applied in the case of combined gas/steam turbine plants. If the plant is, however, relatively often driven at partial load, it follows that regulating the back pressure in this manner is thermodynamically unfavorable since the stack temperature then rises rather sharply.
The additional exhaust gas quantity could be utilized by a downstream-installed low-pressure evaporator during which process steam, is generated and the thermodynamic efficiency of the plant could thereby be improved. On the whole, the total efficiency is, however, poorer is spite of this arrangement since the generated low-pressure steam is of little value.
A primary object of the present invention is to avoid the abovementioned deficiencies and to achieve an improvement of the thermodynamic efficiency of combined gas/steam turbine plants of the abovementioned type.